[JAVA][NATIVE] Fix NPE for form values request builder

[RInverid]

Skip null form values, so no empty fields are sent. Before this change, null values were not an option (that would cause an NPE). The best could be an empty string.

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@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608

[RInverid]

@wing328 any chance this can be looked at before the 6.5.0 release?

[wing328]

LGTM. Thanks for the fix.